The present invention relates generally to internal combustion engines, and specifically to a specialized configuration for a combustion chamber used on such an engine which is designed to minimize spark plug fouling.
Spark plug fouling can be caused when oil, gasoline, or mixtures of the two directly contact the spark plug electrodes to the extent that firing of the electrodes are interrupted or prevented entirely. The problem of fouling is a major design consideration in two-cycle engines, wherein the combustion charge is composed of a fuel-oil mixture, and where the electrodes are located in the cylinder in a position which is exposed to the flow of fresh charge or incompletely combusted fuel, oil and air.
The fouling problem is even more troublesome in small displacement two-cycle outboard marine engines of the type often used to power small boats. This latter class of engines is popular with recreational fishermen, who run their engines for prolonged periods of time at slow or idle speeds, during which the spark plug temperature does not reach the level where excess deposits of excess fuel/oil mixture can be burned off. This type of use exacerbates the spark plug fouling problem.
Small displacement two-cycle engines are often manufactured in the cross-flow format, wherein the fresh charge of gasoline, oil and air enters the cylinder through an intake port virtually simultaneously with the discharge of spent exhaust gases from an exhaust port. In these engines, the intake and exhaust ports are disposed in diametrically opposed locations of the cylinder. To prevent the mixing of fresh charge with the exhaust gases, and to reduce the passage of noncombusted fresh charge out the exhaust port prior to combustion, cross-flow engines are provided with a piston having a top-mounted deflector formation in the shape of a vane or baffle. This vane directs the fresh charge toward the spark plug electrodes, and prevents the fresh charge from passing through the exhaust port prior to combustion.
Cross-flow engines are quite suitable for outboard engines, in that the cross-flow design is particularly effective at "scavenging" the fresh charge gases at low rpm or at idle, when the volume of fresh charge in the combustion chamber is relatively small. This is because the movement of the piston in a two-cycle engine draws the fresh charge into the combustion chamber from the crankcase.
Another operational by-product of a two-cycle engine at idle is that the fresh fuel charge is not fully atomized by the action of the piston, connecting rod and crank shaft on the gas volume in the crankcase. Thus, when these relatively larger droplets of a fresh fuel charge are introduced into the combustion chamber, the potential for fouling increases.
It can be seen that the two-pronged problems of low spark plug temperature and incomplete atomization of the fresh charge in an idling two-cycle engine are significant contributors to the fouling problem. This problem is an especially acute one for sport fisherman, who depend on such small engines for transportation to and from the fishing grounds, and who are inconvenienced in a major way from engine fouling, especially when fouling occurs in remote locations or in foul weather.
One attempt at minimizing the spark plug fouling problem in two-cycle cross-flow engines is disclosed in commonly-assigned U.S. Pat. No. 4,146,004, wherein the cylinder head is provided with a generally V-shaped deflector extending towards the combustion chamber from the internal surface of the head. The deflector is intended to divert the incoming flow of fresh charge away from the spark plug electrodes, and to shield the electrodes from deposits of oil and/or gasoline, or from the impact of the combustion gas flow. Although this design reduced fouling over prior designs, the problem of engine fouling has not been eliminated.
Thus, a primary object of the present invention is to provide a combustion chamber shape for an internal combustion engine in which spark plug fouling is reduced significantly from conventional levels.
Another object of the present invention is to provide a cylinder head for a two-cycle cross-flow type engine having a combustion chamber shape in which the spark plug electrodes are protected from exposure to the direct flow of the fresh charge, but which still provides adequate power to the engine.